The polymerization of conjugated dienes and monovinylarenes under solution conditions with a hydrocarbyl alkali metal compound is known. The use as coupling agents of esters, such as those prepared by the reaction of a carboxylic acid with a hydrocarbyl alcohol, has been a useful procedure to provide a coupled product of improved properties. However, one of the shortcomings of using such esters as coupling agents is that an organic by-product forms, i.e. an alcohol, as a result of the coupling reaction. This by-product often is troublesome if the polymerization diluent is to be recycled, and it furthermore contributes to unfavorable economics of the overall process.
U.S. Pat. No. 3,627,745 discloses the termination of alkali metal initiated diene polymerizations with certain lactones to increase polymer Mooney viscosity, decrease polymer coldflow, increase green tensile strength, and the like. However, U.S. Pat. No. 3,627,745 plainly excludes the use of those lactones derived from gamma-hydroxy acids in the formula shown at col. 1 line 2 in which n=2. 80 J.A.C.S. 6409 (1958) discloses that butyrolactone and other lactones from gamma-hydroxy acids (in the formula shown in U.S. Pat. No. 3,627,745 those in which n would equal 2) do not polymerize due to thermodynamic reasons. Thus, according to the published literature, gamma-hydroxy acid-derived lactones are not useful as polymer terminating agents.